Phenyllactic acid exposure suppresses Microcystis aeruginosa growth with concurrent PSII photochemical impairment and increased extracellular microcystin accumulation

Phenyllactic acid (PLA) has been reported as a bioactive organic acid with anticyanobacterial potential, but its direct ecotoxicological effects on toxin-producing Microcystis aeruginosa remain insufficiently understood. In this study, purified PLA was used to evaluate its effects on the growth, photosynthetic physiology, antioxidant enzyme response and microcystin distribution of M. aeruginosa FACHB-915. Results indicated that PLA inhibited algal growth in a concentration-dependent manner after 48 h exposure with an estimated IC 50 value of 158.3 mg·L −1 , and high PLA exposure markedly reduced photosynthetic pigment production. Meanwhile, fast chlorophyll a fluorescence analysis further showed that exposure to 150.0 mg·L −1 PLA markedly altered PSII coordination, donor-side electron supply, and acceptor-side electron transport in M. aeruginosa . In addition, 150.0 mg·L −1 PLA decreased total protein content and significantly increased SOD activity, whereas POD activity did not show a parallel increase, suggesting distinct responses between the two assessed antioxidant enzymes. Notably, PLA exposure altered microcystin partitioning, and extracellular microcystin concentration increased significantly from 108.01 ng·mL −1 in the control to 126.48 ng·mL −1 at 150.0 mg·L −1 PLA, representing an approximate increase of 17.1%. These findings demonstrate that the growth suppression under PLA exposure was accompanied by pronounced PSII photochemical disturbance and physiological stress, and extracellular microcystin accumulation was also simultaneously enhanced, highlighting the need for careful evaluation of PLA as a potential anticyanobacterial compound.